Ion implanters are widely used in semiconductor manufacturing to selectively alter conductivity of materials. In a typical ion implanter, ions generated from an ion source are directed through a series of beam-line components which include one or more analyzing magnets and a plurality of electrodes. The analyzing magnets select desired ion species, filter out contaminant species and ions having incorrect energies, also adjusting ion beam quality at a target wafer. Suitably shaped electrodes can be used to modify the energy and the shape of the ion beam.
In production, semiconductor wafers are typically scanned with an ion beam. As used hereinafter, “scanning” of an ion beam refers to the relative movement of an ion beam with respect to a wafer or substrate surface.
The ion beam is typically either a “spot beam” having an approximately circular or elliptical cross section or a “ribbon beam” having a rectangular cross section. For the purpose of the present disclosure, a “ribbon beam” may refer to either a static ribbon beam or a scanned ribbon beam. The latter type of ribbon beam may be created by scanning a spot beam back and forth at a high frequency.
In the case of a spot beam, scanning of a wafer may be achieved by sweeping the spot beam back and forth between two endpoints to form a beam path and by simultaneously moving the wafer across the beam path. Alternatively, the spot beam may be kept stationary, and the wafer may be moved in a two-dimensional (2-D) pattern with respect to the spot beam. In the case of a ribbon beam, scanning of a wafer may be achieved by keeping the ribbon beam stationary and by simultaneously moving the wafer across the ribbon beam. If the ribbon beam is wider than the wafer, the one-dimensional (1-D) movement of the wafer may cause the ribbon beam to cover the entire wafer surface. The much simpler 1-D scanning makes ribbon beam a desired choice for single-wafer ion implantation production.
However, just like spot beams, ribbon beams can suffer from intrinsic non-uniformity problems. A ribbon beam typically consists of a plurality of beamlets, wherein each beamlet may be considered, conceptually, as one spot beam. Though beamlets within a ribbon beam travel in the same general direction, any two beamlets may not be pointing in exactly the same direction. In addition, each beamlet may have its intrinsic angle spread. As a result, during ion implantation with a ribbon beam, different locations on a target wafer may experience different ion incident angles. Furthermore, the beamlets may not be evenly spaced within the ribbon beam. One portion of the ribbon beam where beamlets are densely distributed may deliver a higher ion dose than another portion of the ribbon beam where beamlets are sparsely distributed. Therefore, a ribbon beam may lack angle uniformity and/or dose uniformity.
Although there have been attempts to improve either angle uniformity or dose uniformity of a ribbon beam, an efficient solution is not yet available for providing ribbon beams that meet both dose and angle uniformity requirements for ion implantation production. For example, it is typically required that a ribbon beam should produce, in a wafer plane, a dose uniformity with less than 1% variations together with an angle uniformity with less than 0.5° variations. Such stringent uniformity requirements are difficult to meet since both types of uniformity may be elusive.
In view of the foregoing, it would be desirable to provide a technique for improving uniformity of a ribbon beam which overcomes the above-described inadequacies and shortcomings.